1. Field of the Invention
This invention relates to metal to metal seals, and more particularly, to metal to metal piston seals for surface controlled subsurface safety valves used in the oil and gas industry. One aspect of the invention relates to an improved metal to metal piston seal comprising a seal member adapted to provide sealing engagement with a cooperating annular stop at both low and high pressures.
2. Description of Related Art
Surface controlled subsurface safety valves having pistons with conventional metal to metal piston seals are disclosed, for example, in U.S. Pat. No. 4,945,993, which is incorporated by reference herein. Such valves typically comprise a housing having a longitudinal bore communicating with a well tubing string, a valve closure means biased to seal off the bore, and an operator tube adapted to open the valve closure means in response to control fluid pressure. As shown in U.S. Pat. No. 4,945,993, the control fluid pressure acts on a piston means comprising a seal assembly and a cylindrical rod that are slidably disposed in a small diameter longitudinal bore offset from the main valve bore. The operator tube and piston means are upwardly biased by means such as a coil spring annularly disposed around the operator tube. When control fluid pressure in the small diameter bore is decreased below a preselected value, the biasing means moves the operator tube upward to allow the valve closure means to return to its closed position.
The seal assembly of the piston means comprises seal members adapted to prevent control fluid from leaking past the piston in the small diameter bore and seal members adapted to prevent well fluids from flowing back into the control line. A seal member is desirably provided at the top of the seal assembly to block the leak path from the well bore back into the control line. Because of the high pressures and hostile environments in which the piston seal assemblies must often operate, metal to metal seals are preferred for use as the blocking seals.
Conventional metal to metal seals are generally designed for either low or high pressure applications. In a low pressure environment, less contact stress is provided between the sealing elements, and one must rely on precision machining and fit or else use a material such as an elastomer, a non-elastomer, a softer metal, or the like, that will deform to make the necessary seal. Similarly, in a high pressure environment, precision machining and fit are particularly important with conventional seal designs because softer metals will deform and cannot withstand the contact stresses needed to make the seal.
In the past, piston seal assemblies used in subsurface safety valves such as those disclosed in U.S. Pat. No. 4,945,993 have been provided with upwardly facing seat inserts similar to machined bolts. Such seat inserts have had hemispherical or chamfered sealing surfaces adapted to provide metal to metal sealing engagement with a downwardly facing annular shoulder in the small diameter bore. With the prior art metal to metal seals, however, leakage has sometimes occurred due to surface imperfections, loose tolerances, and the like.
A metal to metal piston seal is therefore needed that can be satisfactorily used at either low or high pressures, and that will accommodate imperfections of fit or finish.